Recently, plastic materials such as NYLON 66 or ABS plastic resin reinforced by carbon or glass fiber have superseded natural woods such as persimmon and cherry tree for so-called "wood" or "driver" type golf club heads. A customary way is to form a cavity in the club head by hollowing out the inside of the club head from its sole plate fixing side and making a light-weight club head by enlarging its cavity.
However, there have been problems in that enlargement of the cavity which causes the thinness of the head face where ball impacts take place, gives undesirable effect on the flying distance of the ball at the time of impact for a deformation of the face plate and reduces the mechanical strength of its face plate.
Although the disclosure of Japanese Utility Model Application No. 52-27370 provides a number of horizontal grooves on the club head from its face side to its back side, this type of gold club head could not reduce so much weight of the club head as expected and the production requires very high workmanship for its complicated form. Also, the disclosure of Japanese Utility Model Application No. 52-11460 provides impact core uniform from its face side to back side on the club head, but this type of club head did not meet the reduction of its weight because it must make the head face rather thick for preventing its face side from deformation and, moreover, because of the unified formation, it had a shortcoming that could not especially reinforce the front impact core.